Rewrite pandoc-set-output-formats

Use of let is unnecessary.
Add variable pandoc-extensions
Add space in Literal Haskell menu option (both input and output formats)
Add extensions functionality
Merge branch 'extensions'

Conflicts:
	pandoc-mode.el
Update calls to pandoc-get / pandoc-set in code merged from the extensions branch
Update version number in code and manual.
Make Extensions submenu invisible when in/output formats aren't markdown
Update manual with Extensions submenu.
Merge branch 'extensions'
README.md: Add link to the Pandoc website
README.md: Fix capitalization and formulation